1. Field of the Invention:
The present invention is directed to processes and apparatuses for embrittling and removing an outer protective coating of a pipe or pipeline, and in particular processes and apparatuses for the continuous embrittlement and removal of such outer protective coatings at a speed matching that of typical pipeline traveling coating removing equipment.
2. Brief Description of the Related Art:
In maintaining and repairing pipelines, it is often necessary to remove the protective coating which surrounds the pipes constituting the pipeline. Simply scraping off the coating is both laborious and not fully effective because of the tacky nature of those coatings at the usual temperatures at which the cleaning operations are performed (for example, in South and Southwest USA, summer months), and so it has been proposed to cool the coating to a temperature where the coating is brittle prior to attempting to remove the coating.
An example of a method and apparatus for embrittlement of a pipe coating is shown in U.S. Pat. No. 4,487,643 to Ellett. This patent discloses a batch type process and apparatus in which a flexible sleeve is placed on a selected pipe coating section and sealed both along its length and at its ends. The volume defined by the pipe and the sleeve is then filled with the cryogen carbon dioxide (CO.sub.2) in order to cool the coating to its embrittlement temperature, after which embrittled coating is removed by being struck with a hammer.
The technique of Ellett has been found, however, to have several shortcomings. First, Ellett fails to recognize that a complete and even coating embrittlement (i.e., from the outside skin of the coating down to the bonding interface between the coating and the steel pipe) requires that the underlying steel pipe also be at the embrittlement temperature of the coating, since otherwise the steel pipe, which is a substantial heat source, will heat the inner layers of the coating to above the embrittlement temperature before coating removal is completed. However, cooling both the coating and the underlying steel pipe to a temperature below the embrittlement temperature in a short period of time is impractical by use of a CO.sub.2 cryogen. Because carbon dioxide has a triple point above atmospheric pressure, the enclosure within the sleeve of Ellett must be pressurized to above 100 psig in order to avoid the creation of a block of carbon dioxide snow within the enclosure. Such snow would form a stable calefaction layer around the coated pipe whose insulating properties, together with the insulating properties of the coating itself, would make the rapid cooling of the pipe impossible. Moreover, the gradual vaporization of the CO.sub.2 snow would form gas pockets within the sealed enclosure of Ellett at locations far from the exhaust ports thereof and separated from these ports by the CO.sub.2 snow. There would result increased local pressures within the enclosure which might locally rupture the enclosure.
Although the use of other cryogens, particularly very low temperature cryogens such as liquid nitrogen (LN2 having a vaporization temperature at -196.degree. C.) for cooling an organic coating prior to removal from an object is known, for example from U.S. Pat. No. 3,934,379, the use of such other cryogens would not be obvious from Ellett because the low temperatures would embrittle the synthetic materials of the sleeve itself.
Finally, Ellett has limited practical utility because it is a batch type process which can be used to embrittle and then remove the coating of only one section of a pipe at a time; the embrittlement of the coating of another pipe section would require the removal of the sleeve and repositioning of the same at another location. One could not use the sleeve of Ellett for continuous embrittlement of a pipe coating both because of the need to apply and reapply the sleeve at different locations, because Ellett provides no teaching as to design parameters necessary for continuous embrittlement, i.e., the length of the enclosure, the height of the enclosure, etc., and because Ellett provides no teaching as to necessary related equipment such as the movement driving force for the sleeve if it were continuously movable and for the cryogen source, the interconnection between scraping machine and sleeve, etc.